Conventionally, some diesel engines have a selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a feature of selectively reacting NOx, with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of and is reacted on the reduction catalyst with NOx, (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of the discharged NOx.
Effectiveness of ammonia (NH3) used as a reducing agent for depuration through reduction of NOx, is well known in a field of industrial flue gas denitration in, for example, a plant. However, in a field of automobile where safety is hard to assure as to running with toxic ammonia being loaded, researches have been made aware nowadays on use of nontoxic urea water as a reducing agent (see, for example, Patent Literature 1).
Specifically, urea water added to exhaust gas upstream of a selective reduction catalyst is pyrolytically decomposed into ammonia and carbon dioxide gas in the exhaust gas so that NOx, in the exhaust gas is favorably depurated through reduction on the reduction catalyst by ammonia.
Use of such urea water as the reducing agent requires exhaust temperature of about 200° C. or more so as to exert sufficient catalytic activity in the reductive reaction. Thus, there is a problem that, if an operating status with exhaust temperature of lower than 200° C. continues (generally speaking, low-load operational areas are areas with low exhaust temperature), NOx, reduction ratio is hard to enhance. For example, in a vehicle such as a city shuttle-bus with travel pattern of almost always traveling on congested roads, an operation at a required exhaust temperature or more may not continue for a long time and operational transitions may occur with no chance of NOx reduction ratio being enhanced, failing in obtaining satisfactory NOx, reduction effect.
In order to overcome this, the inventors came to devise incorporation of an oxidation catalyst in the exhaust pipe upstream of the added position of the urea water; when the exhaust gas does not yet reach a lower active limit temperature of the selective reduction catalyst, fuel is added in the exhaust pipe upstream of the urea water added position to bring about oxidation reaction of the added fuel through the oxidation catalyst, the exhaust gas being elevated in temperature by the resultant reaction heat to a required temperature for temperature elevation of a catalyst floor of the selective reduction catalyst. The inventors are further considering that the oxidation catalyst is made to have a property of physically adsorbing NOx, in the exhaust gas to attain NOx reduction through adsorption of NOx, by the oxidation catalyst in operational areas where the temperature of the exhaust gas is lower than the lower active limit temperature of the selective reduction catalyst.
[Patent Literature 1] JP 2002-161732A